Many that ride road and mtb want close to the same posture nominally on each bike. I ride with a 575mm eff. top tube on the road and 625mm on dirt which is a Trek 19.5. The reason for the difference if opting for essentially the same position can be easily explained. With mtb handlebars, they sweep slightly back from the stem clamp centerline where as on the road hood position is about 80mm in front of the stem clamp center. Also, most ride with their bars a bit higher on the dirt. Rule of thumb is mtb's have a 4-6cm longer tt compared to an equivalent road bike size.
HTH.